The present disclosure relates to an image reading apparatus and a foreign matter detection method, and more particularly to an image reading apparatus in which foreign matter that has adhered to the read position of an image obtained with a certain line sensor, from among the line sensors provided for all color components, can be adequately detected, and also to a foreign matter detection method for such an apparatus.
In an image reading apparatus such as a digital copier, a scanner, and a facsimile apparatus, two methods for reading a document image are mainly used. With the first method, a document is fixed to a placement position by placing on a glass table and the image is read by moving an optical system device (configuration with a movable optical system). With the second method, the position of the optical system device is fixed and the image is read, while the document is conveyed by a document conveying device (ADF: Auto Document Feeder) (configuration with a movable document).
In the configuration with a movable document, the read position during reading of an image is fixed at all times on the document table glass. Therefore, a problem occurring when adherent matter such as dust adheres to the document table glass when a document image is read with the configuration with a movable document is that the adhered matter becomes a streaky image in the sub scanning direction and appears in the image that has been read.
In order to resolve this problem, the image reading apparatus using the conventional technique is provided with a provisional foreign matter region detection means and a foreign matter determination means. The provisional foreign matter region detection means takes a pixel, for which the absolute value of the first-order derivative of image data of one color component selected from among the image data of each color component after the inter-line correction is above first reference value, as the lead pixel of a provisional foreign matter region. Then, the provisional foreign matter region detection means takes a pixel with image data equal to or greater than the image data of the lead pixel, from among the image data of the selected one color component, as the end pixel of the provisional foreign matter region. The provisional foreign matter region detection means thus detects the provisional foreign matter region. The foreign matter determination means determines whether or not a difference between the image data corresponding to the lead pixel of the provisional foreign matter region, from among the image data of other color components, and the average value of the image data corresponding to the provisional foreign matter region is less than a second reference value. By using the determination result, the foreign matter determination means determines whether or not foreign matter is present at the read position of the image obtained with a line sensor that has read the image data of the selected one color component.
With such image reading apparatus, whether the provisional foreign matter region detected from the image data of one color component is a region corresponding to foreign matter or a region corresponding to an image can be determined on the basis of density variations in the image data of other color components.
However, when the document including a high-density image is read and the detection of foreign matter that has adhered to the read position corresponding to the line sensors of some color components is performed by using the conventional technique, since the density variation amount caused by the foreign matter is small in the high-density image, it is sometimes impossible to distinguish clearly between the state in which density variations are caused by foreign matter and the case in which density variations are caused by the image design. As a result, a region corresponding to the original image can be erroneously determined as a region corresponding to foreign matter.
Accordingly, it is an object of the present disclosure to reduce the probability of erroneously detecting foreign matter that has adhered to the read position of a line sensor in an image reading apparatus.